Developments in miniaturization and large-scale integration in fluidics have led to the concept of creating an entire chemistry or biology laboratory on a fluidic analog of the electronic microchip. Such integrated microfluidic devices (known as Micro Total Analysis Systems, or μTAS) are seen as key to automating and reducing costs in many biological analysis applications, including genetic analyses and medical diagnostics. Devices for these applications have been fabricated by etching small glass plates or silicon chips or by injection molding or by hot embossing of either hard or soft polymeric materials. Producing reliable valves has proven to be problematic with all of these devices.
Traditional fluid valves operate by moving solid objects to obstruct the flow path. This requires sealing against a valve seat, and often leads to complicated geometries. In etched solid chips, the valves tend to be very complicated, requiring multiple etching and deposition steps, and such valves suffer from a tendency to leak. Valves are easier to make in soft materials but, typically, they have been actuated only by pneumatic pressure. While this method has been shown to work for some applications, there are issues with the large number of pneumatic control lines required for large-scale integrated μTAS devices, as well as with concerns about leakage and the limitations on operating pressure. In addition, this is a normally open valve and can be kept closed only with the continuous application of external pneumatic pressure.
An alternative to moving a solid object to obstruct the flow is to use a phase-change material. A warm liquid is injected into a flow channel where it cools and solidifies, blocking the channel. The channel is reopened by heating and melting the phase-change material. A challenge with this type of valve is providing a source and a sink for the phase-change material. The methods proposed to date include injecting the phase change material from a side channel to close a channel, and letting it collect in a trap downstream of the valve after it is melted to open the channel. A problem with this method is that the phase-change material is used only once before being discarded making it difficult to fabricate multiple-use valves. Another problem is that it is difficult to ensure that all the phase-change material is trapped downstream of the valve, leading to the possibility that solid particles of phase-change material will flow through the rest of the microfluidic device contaminating samples or blocking flow channels.
A valve that avoids some of these problems is the Peltier-actuated valve, in which a phase change is induced in the working fluid itself. This valve is actuated by running an electric current through a Peltier junction adjacent to the fluid flow path. The Peltier junction cools the fluid sufficiently to freeze it. The valve is opened by reversing the current in the Peltier junction to heat the fluid. In a valve operating on this principle there is no contamination problem because the plug is formed from the very fluid the valve is meant to block. When the valve is opened the working fluid is restored to its original liquid state. However, as with the pneumatic valve, this valve also suffers from the drawback of being normally open; power is required to maintain the valve in a closed position. The Peltier-actuated valve is therefore not useful for applications where a flow channel needs to remain closed for extended periods, or in a power-off condition.
Thus, there remains a need for a bi-stable phase-change valve that can remain in either the open or closed position, and in which there is a very low probability of the phase-change material being lost from the valve.